The invention relates to a method and a device for determining a temporal reference point when receiving a radio signal, and in particular, to determining a temporal reference point in a mobile radio system, the radio signal being received with a time-variant received strength in a temporally indeterminate manner.
Conventionally, transmitting communication information via radio channels in radio communication systems or in radio systems is well known, respectively. In a special radio system, a mobile radio system according to the GSM (Global System for Mobile Communication) standard, the information transmitted during the radio transmission is transmitted clustered in the form of so-called bursts. Each information burst has 142 binary symbols to be evaluated, of which normally the center 26 symbols in each case represent the so-called training sequence or midamble, respectively. The training sequence includes transmission information known to the receiver. The training sequence is usually used by the receiving station for determining the respective transmission conditions during the radio transmission and, respectively, eliminating disturbing influences of unfavorable transmission conditions. This makes it possible to correct the actual transmitted information which, for example, is received distorted by linear distortion or dispersion.
It is well known that the radio signal is distorted to a greater or lesser extent depending on the type of terrain or space between the transmitter and the receiver of a radio signal. In particular, multipath propagation occurs, i.e. the transmitted radio signal is reflected, for example from houses, hills or other forms of terrain and reaches the receiver over a number of paths with different delay times. The result is a temporally indeterminate received signal. Furthermore, the individual parts of the received radio signal are received with different received strength. Received strength is understood to be any suitable measure for the strength of the received radio signal, for example the received strength, the amplitude of the received field strength or the signal-to-noise ratio. Another possible cause for the difference between the received radio signal and the transmitted radio signal are interference signals which are received via the same receiving device.
It is also known to transmit to the receiver, at the beginning or repeatedly at relatively great intervals during the radio transmission, special radio information. The radio information allows the receiver to draw conclusions about the respective time at which the training sequence of future received bursts can be expected in each case. The transmission and evaluation of this special radio information which is transmitted, for example, in special bursts, is not without errors since the successive bursts do not arrive at precisely equidistant time intervals at the receiver. Additionally, the incoming radio signals are received in a temporally indeterminate manner as has already been explained, and a time window for receiving the expected training sequence, which is wider than the length of the training sequence, is usually established for each burst in the receiver. An evaluating device in the receiver outputs an output signal at least within the period in which the reception of the training sequence is expected. Since the radio signal and especially the training sequence is received in a temporally indeterminate manner, the output signal, especially the received strength averaged over the length of a training sequence, is, accordingly, output not only for a particular time but for a particular period.
Such an evaluation of the received radio signal for the purpose of correction or for other purposes, as described above, can take place analogously in any radio system in which the transmitted radio information itself includes reference points for the evaluation which can be evaluated. Apart from the evaluation of training sequences, the redundancy of received radio information, which is known in the receiver, can also be used for this purpose, for example. Training sequences are, therefore, not absolutely necessary.
A radio system of another type in accordance with the GSM standard, in which training sequences and/or redundant transmission information can be analogously evaluated is, for example, the future UMTS (Universal Mobile Telecommunication System). Both in GSM and in UMTS, discrete information, i.e. digital data, is transmitted. However, a corresponding evaluation of information known in the receiver is also possible in other systems.
It has already been proposed, for different reasons, to attempt to determine a temporal point of reference in the receiver in the case of a radio signal which has been received in a temporally indeterminate manner. A possibility of using the information thus obtained includes determining the local position of a mobile station in a mobile radio system. It has already been proposed for this purpose to determine the position of the mobile station from a plurality of radio signals, which are transmitted by different mutually distant base stations to the mobile station. However, this presupposes that the mobile radio system contains a defined timing pattern, i.e. at least the time difference of the transmitting times is known in the individual base stations. An attempted spatial resolution of 125 m corresponds to a temporal resolution of the received radio signals of about 400 ns. In the case of GSM, this corresponds to about {fraction (1/9)} of one bit period, i.e. the reciprocal value of the transmission rate at which the individual bits are transmitted. Instead of evaluating radio signals on the base stationxe2x80x94mobile station links (downlink), radio signals of uplinks can also be evaluated.
As already described above, it is known to use in the evaluation of training sequences an evaluating device, especially a correlator which outputs an output signal. The output signal includes, in particular, of the received power, in each case calculated from samples over a time interval having the length of one training sequence. Since the training sequence is received in a temporally indeterminate manner owing to, among other things, the different delay times of the individual signal components of the radio signal, the output signal, in this case the received power, is also temporally indeterminate. It has already been proposed to determine the power centroid of the received power individually for each burst to determine a temporal point of reference. In this case, the definition of the power centroid corresponds to the definition of a mass centroid.
It has also been proposed to determine the temporal point of reference from the component of the radio channel which corresponds to a propagation of the radio signal over the direct path between transmitter and receiver. However, the component of the direct path can frequently only be determined with difficulty, depending on the terrain between the transmitter and the receiver. In cities, in particular, there is frequently no propagation of the radio signal over the direct path at all.
In one embodiment of the invention, there is a method for determining a temporal point of reference when receiving a radio signal, the radio signal being received in a temporally indeterminate manner with a time-variant received strength, determining the received strength of the received radio signal as a function of a time of reception in each case for a plurality of evaluation time intervals, a time base being defined repetitively for each of the evaluation time intervals; averaging the received strength over the plurality of evaluation time intervals, the received strengths in each case resulting in a mean time-dependent received strength with respect to individual time bases; and determining the temporal point of reference from the mean time-dependent received strength as a function of time with respect to the repetitive time base.
In one aspect of the invention, the temporal point of reference is defined as the time of a peak of the mean received strength.
In another aspect of the invention, the radio signal having repetitive training sequences with known signal data, wherein in each of the evaluation time intervals, at least a part of one or more of the training sequences is received in the temporally indeterminate manner.
In still another aspect of the invention, the training sequences are in each case expected in at least one repetitive expected-time windows and the received radio signal is evaluated within the expected-time windows so that the transmitted transmission information can then be corrected, such that the evaluation time intervals are within the expected-time windows and coincide with the latter.
In yet another aspect of the invention, the temporal position of the expected-time windows is specified in dependence on the temporal point of reference determined.
In another aspect of the invention, determining the local position of a receiving station for receiving the radio signals, in which a plurality of the temporally indeterminate radio signals is received by different transmitting stations, sending out the radio signals from the transmitting stations at transmitting times, the transmitting-time difference of which is known or determined in the receiving station; and determining the temporal point of reference of the temporally indeterminate radio signals and deriving information on the position of the receiving station based on the determination.
In still another aspect of the invention, before the temporal point of reference is determined, the temporal resolution of the received strength and/or of the mean time-dependent received strength is increased by interpolation.
In another embodiment of the invention, there is a receiving device to receive a temporally indeterminate radio signal with a time-variant received strength; and an evaluating device to determine a temporal point of reference of the received temporally indeterminate radio signal, wherein the evaluating device includes: a received-strength-determining unit to determine the received strength as a function of the time of reception for a plurality of evaluation time intervals, a time base being defined repetitively for each of the evaluation time intervals, an averaging unit to average the received strength over the evaluation time intervals, the received strengths resulting in a mean time-dependent received strength at identical times with respect to the individual time bases; and a time-determining unit to determine the temporal point of reference from the mean received strength as a function of time.
In one aspect of the invention, an interpolation unit to interpolate the received strength as a function of the time of reception and/or the mean time-dependent received strength as a function of time.
In another aspect of the invention, the interpolation unit has a transversal filter.
In still another aspect of the invention, the transversal filter approximately corresponds to an ideal low-pass filter, the cut-off frequency of which is half the bit rate of a digital received signal of the device.
In yet another aspect of the invention, the averaging unit has a plurality of averaging filters, in which the averaging filters process the received strengths at identical times of the evaluation time intervals which are allocated to the averaging filter, and the averaging filters are preceded by a distributor unit to distribute the received-strength values to the respective associated averaging filters.
In another aspect of the invention, the averaging filters are first-order low-pass filters.
In yet another aspect of the invention, the device is a mobile station or base station for a mobile radio system.